trace_printk(9) [centos man page]

TRACE_PRINTK(9) 						   Driver Basics						   TRACE_PRINTK(9)

NAME

       trace_printk - printf formatting in the ftrace buffer

SYNOPSIS

       trace_printk(fmt, ...);

ARGUMENTS

       fmt
	   the printf format for printing

       ...
	   variable arguments

NOTE

       __trace_printk is an internal function for trace_printk and the ip is passed in via the trace_printk macro.

       This function allows a kernel developer to debug fast path sections that printk is not appropriate for. By scattering in various printk
       like tracing in the code, a developer can quickly see where problems are occurring.

       This is intended as a debugging tool for the developer only. Please refrain from leaving trace_printks scattered around in your code.
       (Extra memory is used for special buffers that are allocated when trace_printk is used)

       A little optization trick is done here. If there's only one argument, there's no need to scan the string for printf formats. The trace_puts
       will suffice. But how can we take advantage of using trace_puts when trace_printk has only one argument? By stringifying the args and
       checking the size we can tell whether or not there are args. __stringify((__VA_ARGS__)) will turn into "()" with a size of 3 when there
       are no args, anything else will be bigger. All we need to do is define a string to this, and then take its size and compare to 3. If it's
       bigger, use do_trace_printk otherwise, optimize it to trace_puts. Then just let gcc optimize the rest.

COPYRIGHT

Kernel Hackers Manual 3.10					     June 2014							   TRACE_PRINTK(9)

vprintf(3S)															       vprintf(3S)

NAME

       vprintf(), vfprintf(), vsprintf(), vsnprintf() - print formatted output of a varargs argument list

SYNOPSIS


DESCRIPTION

       and  are  the  same  as	and respectively, except that instead of being called with a variable number of arguments, they are called with an
       argument list as defined by

RETURN VALUE

       Each function returns the number of bytes transmitted (excluding the null byte character in the case of or a negative value  if	an  output
       error was encountered.

       By default, returns a negative value if maxsize is smaller than the number of characters formatted.  In the UNIX 2003 standards environment
       (see standards(5)) it returns the number of bytes that would have been written to buffer s, excluding the terminating null byte, if maxsize
       had been sufficiently large.

EXAMPLES

       The following demonstrates how could be used to write an error routine:

	      #include <stdarg.h>
	      #include <stdio.h>
		  .
		  .
		  .
	      /*
	       *  error should be called using the form:
	       *      error(function_name, format, arg1, arg2...);
	       */

	      /*VARARGS0*/
	      void
	      error(va_alist)
	      va_dcl
	      {
		  va_list args;
		  char *fmt;

		  va_start(args);

		  /* print out name of function causing error */
		  (void)fprintf(stderr, "ERROR in %s: ", va_arg(args, char *));
		  fmt = va_arg(args, char *);

		  /* print out remainder of message */
		  (void)vfprintf(stderr, fmt, args);
		  va_end(args);
		  (void)abort( );
	      }

SEE ALSO

       setlocale(3C), printf(3S), standards(5), thread_safety(5), varargs(5), glossary(9).

STANDARDS CONFORMANCE

																       vprintf(3S)